The present invention relates to a biaxially oriented laminated polyester film and a magnetic recording medium. More specifically, it relates to a biaxially oriented laminated polyester film having excellent anti-block properties, winding properties and processability and a magnetic recording medium comprising the same and having excellent electromagnetic conversion characteristics.
A biaxially oriented polyester film typified by a polyethylene terephthalate film is used for various purposes, particularly as a base film for magnetic recording media, thanks to its excellent physical and chemical properties.
Along with recent efforts made to increase the density and capacity of a magnetic recording medium, a more flat and thinner base film is desired. Particularly a recently developed multi-layer metal magnetic tape having performance equivalent to a deposited magnetic tape requires a base film having extremely high surface flatness.
However, when the surface of a base film is flattened to retain excellent electromagnetic conversion characteristics, its slipperiness or air squeezability deteriorates. As a result, when the film is to be rolled, it tends to be wrinkled or debris, thereby making it extremely difficult to roll the film nicely. When the slipperiness of the base film is low in the film processing step after the film is rolled, the friction of the base film with a metal roll in contact with the base film increases, whereby the film may be wrinkled, a magnetic layer may not be formed smoothly, or the film may not be calendered smoothly.
To improve the slipperiness of a polyester film, the following methods are employed: (1) one in which inert particles are precipitated from the catalyst residue in a raw material polymer in the production process and (2) one in which inert particles are added separate from the catalyst residue. Both methods provide fine irregularities to the surface of the film. As the size of the particles contained in the film increases, larger irregularities are formed on the surface of the film and as the content of the particles grows, a larger number of irregularities are formed on the surface of the film. Therefore, when the size or content of the particles is increased, the surface of the film becomes rough and the slipperiness of the film is improved. However, as described above, the surface of a base film is desired to be as flat as possible in order to improve electromagnetic conversion characteristics. When the surface of a base film is rough and a magnetic recording medium is formed using the base film, protrusions on the surface of the base film thrust up the surface of a magnetic layer after the formation of the magnetic layer, thereby deteriorating electromagnetic conversion characteristics. Consequently, the larger the size and content of the particles contained in the base film, the higher the surface roughness of the base film becomes, thereby deteriorating electromagnetic conversion characteristics but improving slipperiness.
As means of improving both slipperiness and electromagnetic conversion characteristics which are antipodal properties, it is proposed to form different surface forms on the two respective sides of the film. Stated more specifically, there is widely known a biaxially oriented laminated polyester film having a flat layer capable of improving electromagnetic conversion characteristics on one side to be coated with a magnetic layer and a roughened layer for improving slipperiness on the opposite side.
However, even when the side (flat side) to be coated with a magnetic layer of the above biaxially oriented laminated polyester film is flattened and the opposite side (to be referred to as xe2x80x9crough sidexe2x80x9d hereinafter) is roughened, if the film is thin, the roughened side exerts an influence upon the magnetic layer forming side (flat side), undulates the flat side and impairs the flatness of the flat side according to the type, size and amount of particles to be contained in the rough side layer. Particularly for the latest high-density magnetic recording medium, a metal calender having a high linear pressure is used to meet demand for a more flat magnetic layer, whereby a problem that particles existent in the above rough side greatly thrust up the flat side to reduce its flatness is becoming very serious.
To reduce the number of protrusions on the flat side thrust up from the rough side, there have been proposed a method for reducing the particle diameter of particles to be contained in the rough side and a method for reducing the amount of particles to be added. However, in the former case, sufficient air squeezability cannot be obtained because the height of protrusions formed on the rough side is low whereas in the latter case, sufficient film slipperiness cannot be obtained because the density of protrusions formed on the rough side is low. When the film is rolled, it is vertically wrinkled in the former case and its surface becomes debris in the latter case, thereby making it impossible to obtain a sufficiently high product yield. Stated more specifically, in order to further improve electromagnetic conversion characteristics, there is proposed a flat layer containing substantially no particles as part of an effort to greatly flatten the magnetic layer forming side of the film. In this case, as the running properties of the flat side are unsatisfactory in the tape forming step, the film is wrinkled in that step and the product yield is greatly reduced.
Thus, a film having very high flatness on the flat side and film slipperiness at the same time has not yet been provided.
When the film is used in a magnetic recording tape of linear recording system, the flattening of the rough side is desired in addition to the flattening of the above flat side and film slipperiness as described above. This is because a countermeasure against track dislocation is necessary along with an increase in the density of tracks and the positioning accuracy of tracks is improved by recording a tracking servo signal on the back coat side which is the rough side. Therefore, when the film is used in a magnetic recording tape of linear recording system, the roughening of the back coat side is restricted and a high level of film slipperiness must be achieved by flattening both the flat side and rough side.
It is an object of the present invention to provide a biaxially oriented laminated polyester film which overcomes the above defects of the prior art, has excellent winding properties and processability through the flattening of the flat side and can exhibit excellent electromagnetic conversion characteristics when a magnetic recording medium, particularly a metal coated high recording density magnetic recording medium is formed therefrom.
It is another object of the present invention to provide a biaxially oriented laminated polyester film which has excellent winding properties and processability while meeting new demand for the flattening of not only the flat side but also the rough side and can exhibit excellent electromagnetic conversion characteristics when a magnetic recording medium, particularly a metal coated high recording density magnetic recording medium is formed therefrom.
It is still another object of the present invention to provide a magnetic recording medium which comprises the biaxially oriented laminated film of the present invention as a base film and has excellent electromagnetic conversion characteristics.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by a biaxially oriented laminated polyester film comprising: (A) a first polyester layer which contains (1) at least three types of inert particles: 0.005 to 0.1 wt % of first inert particles having an average particle diameter of 0.4 to 0.7 xcexcm and a relative standard deviation of particle size distribution of 0.5 or less, 0.05 to 0.3 wt % of second inert particles having an average particle diameter of 0.2 xcexcm or more and less than 0.4 xcexcm and a relative standard deviation of particle size distribution of 0.5 or less, and 0.1 to 0.5 wt % of third inert particles having an average particle diameter of 0.01 xcexcm or more and less than 0.2 xcexcm; and 0.05 to 0.25 wt % of an ester compound of an aliphatic monocarboxylic acid having 12 or more carbon atoms and a polyhydric alcohol, in a ratio satisfying the following expression:
xe2x88x920.5x+0.15xe2x89xa6yxe2x89xa6xe2x88x920.5x+0.3 
wherein x is the content (wt %) of the second inert particles and y is the content (wt %) of the ester compound,
the above three types of inert particles showing three clearly distinguishable particle size peaks within the above average particle diameter ranges in a particle size distribution curve, respectively,
and which has (2) a center plane average surface roughness (WRa) of 5 to 20 nm and a 10-point average surface roughness (WRz) of 100 to 300 nm, and
(B) a second polyester layer having a center plane average surface roughness (WRa) of 1 to 10 nm, formed on the first polyester layer,
and having (C) one of Young""s moduli in lengthwise and crosswise directions of 6 GPa or more and the other Young""s modulus of 4 GPa or more.
According to the present invention, secondly, the above objects and advantages of the present invention are attained by a magnetic recording medium comprising the biaxially oriented laminated polyester film of the present invention and a magnetic layer formed on the surface of the second polyester layer of the biaxially oriented laminated polyester film.
According to the present invention, thirdly, the above objects and advantages of the present invention are attained by a magnetic recording medium comprising the biaxially oriented laminated polyester film of the present invention, a magnetic layer formed on the surface of the second polyester layer of the biaxially oriented laminated polyester film and a layer for recording a tracking servo signal, formed on the surface of the first polyester layer of the biaxially oriented polyester laminated film.